rk = r0 -- sk, eqex(st, st0, r) ⊦ r0 = r, st[rk] = st0[rk]
[[CONCRETESIMPRULES : ([CSIMPELIM : ⊦ r = rk .ref ∧ sk = rk .key ↔ rk = r -- sk])], [EXTRATERMS : (rk, r0, sk)]]